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                <h2 id="FFmpeg-avio-list-dir"><a href="#FFmpeg-avio-list-dir" class="headerlink" title="FFmpeg-avio_list_dir"></a>FFmpeg-avio_list_dir</h2><p>文件夹读取分为4个步骤:</p>
<ol>
<li><p>创建文件读取的上下文对象 <code>URLContext</code>,用来保存记录文件读取过程中所有环境变量和源数据</p>
</li>
<li><p>根据文件查找相对应的文件协议<code>URLProtocol</code>,这里操作的是本地文件,所以是<code>ff_file_protocol</code>对象,不同的文件格式所对应的文件协议的实现也不一样,如<code>ff_https_protocol</code>对文件的操作主要是通过网络处理的</p>
</li>
<li><p>通过<code>URLContext</code>内部的priv_data(FileContext)提供文件的处理方式,如blocSize,seekable</p>
</li>
<li><p>通过URLContext-&gt;port(URLProtocol)操作文件并返回需要的数据</p>
</li>
</ol>
<h2 id="图解"><a href="#图解" class="headerlink" title="图解"></a>图解</h2><p><img src="http://assets.processon.com/chart_image/6054161a1e08535530daf30c.png" alt="list_op图解"></p>
<h2 id="URLContext"><a href="#URLContext" class="headerlink" title="URLContext"></a>URLContext</h2><p>它是一个文件的上下文对象,主要负责提供文件处理过程的状态信息</p>
<figure class="highlight c"><table><tbody><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">typedef</span> <span class="class"><span class="keyword">struct</span> <span class="title">URLContext</span> {</span></span><br><span class="line">    <span class="keyword">const</span> AVClass *av_class;        <span class="comment">//用来提供上下文日志信息的打印,以及提供AVOptions</span></span><br><span class="line">    <span class="keyword">const</span> <span class="class"><span class="keyword">struct</span> <span class="title">URLProtocol</span> *<span class="title">prot</span>;</span> <span class="comment">//文件协议对象,遵守文件的基本协议如`open/close/read`</span></span><br><span class="line">    <span class="keyword">void</span> *priv_data;                <span class="comment">//它是一个私有的文件操作对象,在URLProtocol的具体实现方法中读取并记录文件信息</span></span><br><span class="line">    <span class="keyword">char</span> *filename;                 <span class="comment">//文件的路径信息</span></span><br><span class="line">    <span class="keyword">int</span> flags;                      <span class="comment">//标志位,如`AVIO_FLAG_READ`,`AVIO_FLAG_WRITE`,还有很多，具体搜索`AVIO_FLAG`关键字</span></span><br><span class="line">    <span class="keyword">int</span> max_packet_size;            <span class="comment">//读取的最大Packet Size</span></span><br><span class="line">    <span class="keyword">int</span> is_streamed;                <span class="comment">//是否正在传输数据</span></span><br><span class="line">    <span class="keyword">int</span> is_connected;               <span class="comment">//当打开了文件,持有了文件描述符则标记为链接</span></span><br><span class="line">    AVIOInterruptCB interrupt_callback; <span class="comment">//文件读取被终端</span></span><br><span class="line">    <span class="keyword">int64_t</span> rw_timeout;             <span class="comment">//读取/写入/网络链接 网络链接的超时时间,mcs表示</span></span><br><span class="line">    <span class="keyword">const</span> <span class="keyword">char</span> *protocol_whitelist;  <span class="comment">//白名单协议</span></span><br><span class="line">    <span class="keyword">const</span> <span class="keyword">char</span> *protocol_blacklist;  <span class="comment">//黑名单协议</span></span><br><span class="line">    <span class="keyword">int</span> min_packet_size;             <span class="comment">//同max_packet_size相反</span></span><br><span class="line">} URLContext;</span><br></pre></td></tr></tbody></table></figure>

<h2 id="URLProtocol"><a href="#URLProtocol" class="headerlink" title="URLProtocol"></a>URLProtocol</h2><p>它是一个文件协议对象,定义了文件相关的协议方法,FFmpeg在处理文件时会去全局的文件协议注册表里查找合适的协议来处理文件,在这个example中使用的是ff_file_protocol来操作文件</p>
<figure class="highlight c"><table><tbody><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">const</span> URLProtocol ff_file_protocol = {</span><br><span class="line">    .name                = <span class="string">"file"</span>,</span><br><span class="line">    .url_open            = file_open,</span><br><span class="line">    .url_read            = file_read,</span><br><span class="line">    .url_write           = file_write,</span><br><span class="line">    .url_seek            = file_seek,</span><br><span class="line">    .url_close           = file_close,</span><br><span class="line">    .url_get_file_handle = file_get_handle,</span><br><span class="line">    .url_check           = file_check,</span><br><span class="line">    .url_delete          = file_delete,</span><br><span class="line">    .url_move            = file_move,</span><br><span class="line">    .priv_data_size      = <span class="keyword">sizeof</span>(FileContext),  <span class="comment">//FileContext对应URLContext的priv_data,提供操作的配置选项信息以及保文件描述符</span></span><br><span class="line">    .priv_data_class     = &amp;file_class,          <span class="comment">//为文件提供AVOptions,初始化FileContext</span></span><br><span class="line">    .url_open_dir        = file_open_dir,</span><br><span class="line">    .url_read_dir        = file_read_dir,</span><br><span class="line">    .url_close_dir       = file_close_dir,</span><br><span class="line">    .default_whitelist   = <span class="string">"file,crypto,data"</span></span><br><span class="line">};</span><br></pre></td></tr></tbody></table></figure>

<h2 id="FileContext"><a href="#FileContext" class="headerlink" title="FileContext"></a>FileContext</h2><p>它是根据文件协议的<code>priv_data_size</code>和<code>priv_data_class</code>创建的, <code>priv_data_size</code>定义了它的大小,用于分配空间,<code>priv_data_class</code>则定义了它的AVOptions属性,用于设置<code>FileContext</code>初始化属性,如下: </p>
<figure class="highlight c"><table><tbody><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">typedef</span> <span class="class"><span class="keyword">struct</span> <span class="title">FileContext</span> {</span></span><br><span class="line">    <span class="keyword">const</span> AVClass *<span class="class"><span class="keyword">class</span>;</span>   <span class="comment">//关连的AVClass,此示例中关联的是上文提到的`file_class`,它也是AVClass类型</span></span><br><span class="line">    <span class="keyword">int</span> fd;         <span class="comment">//文件描述符</span></span><br><span class="line">    <span class="keyword">int</span> trunc;      <span class="comment">//尾部 </span></span><br><span class="line">    <span class="keyword">int</span> blocksize;  <span class="comment">//读取的块大小</span></span><br><span class="line">    <span class="keyword">int</span> follow;     <span class="comment">//跟踪正在写入的文件</span></span><br><span class="line">    <span class="keyword">int</span> seekable;   <span class="comment">//设置文件是否可以查找指定位置,游标</span></span><br><span class="line"><span class="meta">#<span class="meta-keyword">if</span> HAVE_DIRENT_H</span></span><br><span class="line">    DIR *dir;       <span class="comment">//调用系统api返回的Dir指针,记录了文件夹信息</span></span><br><span class="line"><span class="meta">#<span class="meta-keyword">endif</span></span></span><br><span class="line">} FileContext;</span><br></pre></td></tr></tbody></table></figure>

<h2 id="URLContext-gt-prvi-data"><a href="#URLContext-gt-prvi-data" class="headerlink" title="URLContext->prvi_data"></a>URLContext-&gt;prvi_data</h2><p>prvi_data对应的是<code>FileContext</code>对象,它描述的是定义文件处理的相关选项信息,如读取块的大小,是否可以查找指定位置,同时也会记录文件描述符,配合<code>URRLProtocol</code>的协议方法处理文件.</p>
<p>URLContext-&gt;prvi_data(FileContext)是如何和<code>ff_file_protocol-&gt;priv_data_class</code>(AVClass)关联的？</p>
<ul>
<li>在通过URLProtocol读取文件文件信息的时候URLContext会指定将<code>priv_data</code>转换为<code>FileContext</code></li>
</ul>
<figure class="highlight c"><table><tbody><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">static</span> <span class="keyword">int</span> <span class="title">file_open_dir</span><span class="params">(URLContext *h)</span></span></span><br><span class="line"><span class="function"></span>{</span><br><span class="line"><span class="meta">#<span class="meta-keyword">if</span> HAVE_LSTAT</span></span><br><span class="line">    FileContext *c = h-&gt;priv_data;</span><br><span class="line">  </span><br><span class="line">    c-&gt;dir = opendir(h-&gt;filename);</span><br><span class="line">    <span class="keyword">if</span> (!c-&gt;dir)</span><br><span class="line">        <span class="keyword">return</span> AVERROR(errno);</span><br><span class="line"></span><br><span class="line">    <span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line"><span class="meta">#<span class="meta-keyword">else</span></span></span><br><span class="line">    <span class="keyword">return</span> AVERROR(ENOSYS);</span><br><span class="line"><span class="meta">#<span class="meta-keyword">endif</span> <span class="comment">/* HAVE_LSTAT */</span></span></span><br><span class="line">}</span><br></pre></td></tr></tbody></table></figure>

<ul>
<li>在初始化<code>URLContext</code>的<code>port</code>(URLProtocol)文件协议对象的时候,会将<code>priv_data</code>和<code>file_class</code>关联,并初始化priv_data(FileContext)中的属性</li>
</ul>
<figure class="highlight c"><table><tbody><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">static</span> <span class="keyword">int</span> <span class="title">url_alloc_for_protocol</span><span class="params">(URLContext **puc, <span class="keyword">const</span> URLProtocol *up,</span></span></span><br><span class="line"><span class="function"><span class="params">                                  <span class="keyword">const</span> <span class="keyword">char</span> *filename, <span class="keyword">int</span> flags,</span></span></span><br><span class="line"><span class="function"><span class="params">                                  <span class="keyword">const</span> AVIOInterruptCB *int_cb)</span></span></span><br><span class="line"><span class="function"></span>{  ...</span><br><span class="line">    <span class="keyword">if</span> (up-&gt;priv_data_size) { </span><br><span class="line">         ...</span><br><span class="line">            *(<span class="keyword">const</span> AVClass **)uc-&gt;priv_data = up-&gt;priv_data_class;</span><br><span class="line">            .... </span><br><span class="line">            av_opt_set_defaults(uc-&gt;priv_data);</span><br></pre></td></tr></tbody></table></figure>

<ul>
<li><code>file_class</code>同时也提供的<code>AVOptions</code>设置到<code>FileContext</code>中</li>
</ul>
<figure class="highlight c"><table><tbody><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">static</span> <span class="keyword">const</span> AVClass file_class = {</span><br><span class="line">    .class_name = <span class="string">"file"</span>,</span><br><span class="line">    .item_name  = av_default_item_name,</span><br><span class="line">    .option     = file_options,</span><br><span class="line">    .version    = LIBAVUTIL_VERSION_INT,</span><br><span class="line">};</span><br><span class="line"><span class="keyword">static</span> <span class="keyword">const</span> AVOption file_options[] = {</span><br><span class="line">    { <span class="string">"truncate"</span>, ...</span><br><span class="line">    { <span class="string">"blocksize"</span>,  ...</span><br><span class="line">    { <span class="string">"follow"</span>,  ...</span><br><span class="line">    { <span class="string">"seekable"</span>, ...</span><br><span class="line">};</span><br></pre></td></tr></tbody></table></figure>

<ul>
<li><p>priv_data_class是一个可选配置,此处是为了给这个文件操作的上下文<code>URLContext-&gt;priv_data(FileContext)</code>设置file文件的默认参数,其实就是初始化附值</p>
</li>
<li><p>除了默认的初始化复制,还可以通过外界传入 <code>AVOptions</code>来覆盖默认的初始化赋值</p>
</li>
</ul>
<figure class="highlight c"><table><tbody><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">int</span> <span class="title">avio_open_dir</span><span class="params">(AVIODirContext **s, <span class="keyword">const</span> <span class="keyword">char</span> *url, AVDictionary **options)</span></span></span><br><span class="line"><span class="function"></span>{</span><br><span class="line">    URLContext *h = <span class="literal">NULL</span>;</span><br><span class="line">    AVIODirContext *ctx = <span class="literal">NULL</span>;</span><br><span class="line">    ...</span><br><span class="line">    <span class="keyword">if</span> (h-&gt;prot-&gt;url_open_dir &amp;&amp; h-&gt;prot-&gt;url_read_dir &amp;&amp; h-&gt;prot-&gt;url_close_dir) {</span><br><span class="line">        <span class="keyword">if</span> (options &amp;&amp; h-&gt;prot-&gt;priv_data_class &amp;&amp;</span><br><span class="line">            (ret = av_opt_set_dict(h-&gt;priv_data, options)) &lt; <span class="number">0</span>)</span><br></pre></td></tr></tbody></table></figure>


<h2 id="AVClass的作用"><a href="#AVClass的作用" class="headerlink" title="AVClass的作用"></a>AVClass的作用</h2><p>在这个文件信息读取的示例中,已有2个类第一个属性为AVClass<br>URLContext - AVClass:<br>FileContext - AVClass: 这里关联的是 file_class,为FileContext提供了默认的AVOptions,初始化文件读写的一些属性，如blocSize,seekable,truncate</p>
<figure class="highlight c"><table><tbody><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br><span class="line">55</span><br><span class="line">56</span><br><span class="line">57</span><br><span class="line">58</span><br><span class="line">59</span><br><span class="line">60</span><br><span class="line">61</span><br><span class="line">62</span><br><span class="line">63</span><br><span class="line">64</span><br><span class="line">65</span><br><span class="line">66</span><br><span class="line">67</span><br><span class="line">68</span><br><span class="line">69</span><br><span class="line">70</span><br><span class="line">71</span><br><span class="line">72</span><br><span class="line">73</span><br><span class="line">74</span><br><span class="line">75</span><br><span class="line">76</span><br><span class="line">77</span><br><span class="line">78</span><br><span class="line">79</span><br><span class="line">80</span><br><span class="line">81</span><br><span class="line">82</span><br><span class="line">83</span><br><span class="line">84</span><br><span class="line">85</span><br><span class="line">86</span><br><span class="line">87</span><br><span class="line">88</span><br><span class="line">89</span><br><span class="line">90</span><br><span class="line">91</span><br><span class="line">92</span><br></pre></td><td class="code"><pre><span class="line">用于描述一个AVClass上下文的结构体对象这是一个抽象的结构体对象,第一个属性只想结构体本身,其它的 XXContext类第一个属性也是 AVClass就遵守了这个AVClass协议,基于首地址指针相同的原理,Context和AVClass可以进行类型互转,从而很方便的进行<span class="built_in">log</span>输出以及从AVClass中设置数据(AVOptions)设置</span><br><span class="line"></span><br><span class="line">从Dart语言设计角度来看,它就像是给 Context对象加了一个 mixin的方法并提供了一个抽象类.</span><br><span class="line"></span><br><span class="line"><span class="comment">/**</span></span><br><span class="line"><span class="comment"></span></span><br><span class="line"><span class="comment"> * Describe the class of an AVClass context structure. That is an</span></span><br><span class="line"><span class="comment"> * arbitrary struct of which the first field is a pointer to an</span></span><br><span class="line"><span class="comment"> * AVClass struct (e.g. AVCodecContext, AVFormatContext etc.).</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"><span class="keyword">typedef</span> <span class="class"><span class="keyword">struct</span> <span class="title">AVClass</span> {</span></span><br><span class="line">    <span class="comment">/**</span></span><br><span class="line"><span class="comment">     * The name of the class; usually it is the same name as the</span></span><br><span class="line"><span class="comment">     * context structure type to which the AVClass is associated.</span></span><br><span class="line"><span class="comment">     */</span></span><br><span class="line">    <span class="keyword">const</span> <span class="keyword">char</span>* class_name;  </span><br><span class="line"></span><br><span class="line">    <span class="comment">/**</span></span><br><span class="line"><span class="comment">     * A pointer to a function which returns the name of a context</span></span><br><span class="line"><span class="comment">     * instance ctx associated with the class.</span></span><br><span class="line"><span class="comment">     */</span> </span><br><span class="line">    <span class="keyword">const</span> <span class="keyword">char</span>* (*item_name)(<span class="keyword">void</span>* ctx);</span><br><span class="line"></span><br><span class="line">    <span class="comment">/**</span></span><br><span class="line"><span class="comment">     * a pointer to the first option specified in the class if any or NULL</span></span><br><span class="line"><span class="comment">     *</span></span><br><span class="line"><span class="comment">     * @see av_set_default_options()</span></span><br><span class="line"><span class="comment">     */</span></span><br><span class="line">    <span class="keyword">const</span> <span class="class"><span class="keyword">struct</span> <span class="title">AVOption</span> *<span class="title">option</span>;</span></span><br><span class="line"></span><br><span class="line">    <span class="comment">/**</span></span><br><span class="line"><span class="comment">     * LIBAVUTIL_VERSION with which this structure was created.</span></span><br><span class="line"><span class="comment">     * This is used to allow fields to be added without requiring major</span></span><br><span class="line"><span class="comment">     * version bumps everywhere.</span></span><br><span class="line"><span class="comment">     */</span></span><br><span class="line"></span><br><span class="line">    <span class="keyword">int</span> version;</span><br><span class="line"></span><br><span class="line">    <span class="comment">/**</span></span><br><span class="line"><span class="comment">     * Offset in the structure where log_level_offset is stored.</span></span><br><span class="line"><span class="comment">     * 0 means there is no such variable</span></span><br><span class="line"><span class="comment">     */</span></span><br><span class="line">    <span class="keyword">int</span> log_level_offset_offset;</span><br><span class="line"></span><br><span class="line">    <span class="comment">/**</span></span><br><span class="line"><span class="comment">     * Offset in the structure where a pointer to the parent context for</span></span><br><span class="line"><span class="comment">     * logging is stored. For example a decoder could pass its AVCodecContext</span></span><br><span class="line"><span class="comment">     * to eval as such a parent context, which an av_log() implementation</span></span><br><span class="line"><span class="comment">     * could then leverage to display the parent context.</span></span><br><span class="line"><span class="comment">     * The offset can be NULL.</span></span><br><span class="line"><span class="comment">     */</span></span><br><span class="line">    <span class="keyword">int</span> parent_log_context_offset;</span><br><span class="line"></span><br><span class="line">    <span class="comment">/**</span></span><br><span class="line"><span class="comment">     * Return next AVOptions-enabled child or NULL</span></span><br><span class="line"><span class="comment">     */</span></span><br><span class="line">    <span class="keyword">void</span>* (*child_next)(<span class="keyword">void</span> *obj, <span class="keyword">void</span> *prev);</span><br><span class="line"></span><br><span class="line">    <span class="comment">/**</span></span><br><span class="line"><span class="comment">     * Category used for visualization (like color)</span></span><br><span class="line"><span class="comment">     * This is only set if the category is equal for all objects using this class.</span></span><br><span class="line"><span class="comment">     * available since version (51 &lt;&lt; 16 | 56 &lt;&lt; 8 | 100)</span></span><br><span class="line"><span class="comment">     */</span></span><br><span class="line">    AVClassCategory category;</span><br><span class="line"></span><br><span class="line">    <span class="comment">/**</span></span><br><span class="line"><span class="comment">     * Callback to return the category.</span></span><br><span class="line"><span class="comment">     * available since version (51 &lt;&lt; 16 | 59 &lt;&lt; 8 | 100)</span></span><br><span class="line"><span class="comment">     */</span></span><br><span class="line">    AVClassCategory (*get_category)(<span class="keyword">void</span>* ctx);</span><br><span class="line"></span><br><span class="line">    <span class="comment">/**</span></span><br><span class="line"><span class="comment">     * Callback to return the supported/allowed ranges.</span></span><br><span class="line"><span class="comment">     * available since version (52.12)</span></span><br><span class="line"><span class="comment">     */</span></span><br><span class="line">    <span class="keyword">int</span> (*query_ranges)(struct AVOptionRanges **, <span class="keyword">void</span> *obj, <span class="keyword">const</span> <span class="keyword">char</span> *key, <span class="keyword">int</span> flags);</span><br><span class="line"></span><br><span class="line">    <span class="comment">/**</span></span><br><span class="line"><span class="comment">     * Iterate over the AVClasses corresponding to potential AVOptions-enabled</span></span><br><span class="line"><span class="comment">     * children.</span></span><br><span class="line"><span class="comment">     *</span></span><br><span class="line"><span class="comment">     * @param iter pointer to opaque iteration state. The caller must initialize</span></span><br><span class="line"><span class="comment">     *             *iter to NULL before the first call.</span></span><br><span class="line"><span class="comment">     * @return AVClass for the next AVOptions-enabled child or NULL if there are</span></span><br><span class="line"><span class="comment">     *         no more such children.</span></span><br><span class="line"><span class="comment">     *</span></span><br><span class="line"><span class="comment">     * @note The difference between child_next and this is that child_next</span></span><br><span class="line"><span class="comment">     *       iterates over _already existing_ objects, while child_class_iterate</span></span><br><span class="line"><span class="comment">     *       iterates over _all possible_ children.</span></span><br><span class="line"><span class="comment">     */</span></span><br><span class="line">    <span class="keyword">const</span> <span class="class"><span class="keyword">struct</span> <span class="title">AVClass</span>* (*<span class="title">child_class_iterate</span>)(<span class="title">void</span> **<span class="title">iter</span>);</span></span><br><span class="line">} AVClass;</span><br></pre></td></tr></tbody></table></figure>


                
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